Earlier this year, I replaced my PB LotR trilogy film tie-in editions with ones which match the other PB set we've got. Unfortunately, there was an inexplicable size difference in vol 2-3. Just found a TTT at a local charity shop that matches. £1.60 bargain!

If you look at the Two Towers, on Tolkienbooks.net, there are at least 4 versions

Yeah... so HarperCollins published these in Canada as well (they were printed in the U.S. but not released there). Nearly all of these HarperCollins Canadian 'editions' are slightly smaller than their U.K. counterparts, for whatever reason.